Abstract

Introductory Sentence indicating purpose of study: Circulating tumor cell (CTC) clusters have been shown to have higher metastatic potential than single CTCs in breast, pancreatic, and other cancers, yet these clusters have not been extensively described in metastatic castrate-resistant prostate cancer (MCRPC). Here we sought to determine the feasibility of capturing and characterizing CTC clusters in prostate cancer patients.

Description of Experimental Procedures: Fifty-five blood samples from 29 MCRPC patients, ages 50 to 81 (median age 68), were obtained prior to the patient starting or switching to androgen receptor inhibitor or 17 alpha lyase inhibitor therapies. Eighteen patients received enzalutamide and 11 received abiraterone. The majority of patients had a Gleason score > 7 (22; 75.9%), bone metastases (19; 65.5%), and an ECOG status of 0 (21; 72.4%). All patients had previously undergone, or were receiving at time of enrollment, androgen deprivation therapy. Eleven patients (37.9%) had been on a prior 2nd generation anti-androgen therapy. CTC single cells and clusters (2 or more cells together in one image) were enumerated using the CellSearch system and stained to detect expression of androgen receptor (AR), glucocorticoid receptor (GR), and neuroendocrine (NE) markers.

Summary of Data: Five or more single CTCs, a measure which has previously been associated with an unfavorable prognosis, were detected in 13 of 29 patients (44.8%), and in 19 of 55 blood samples (34.5%). Altogether, a total of 282 CTC clusters was detected, with 1 or more clusters found in 10 patients (34.5%) and 13 samples (23.6%). The number of clusters per 7.5ml of blood ranged from 0-150, and clusters contained anywhere from 2 to 16 cells. Most CTC clusters (268; 95.0%) contained only CTCs and no leukocytes. Just over half the detected clusters (162; 57.4%) contained only 2 CTCs. Cluster staining patterns were fairly homogenous with 29.4% of clusters having uniform expression of either AR, GR, or NE markers, i.e., all CTCs in the cluster expressed the marker of interest. Most clusters (67.7%) were uniformly marker negative and the remaining 2.8% demonstrated a mix of marker positive and marker negative CTCs. Serum Chromogranin A levels, as determined by standard of care clinical blood testing, were found to be positively associated with the number of CTC clusters per 7.5ml of blood (p<0.0001).

Statement of Conclusions: The capture and characterization of CTC clusters in the blood of MCRPC patients can be successfully performed using the CellSearch system. Further investigation into the clinical implications of these clusters is warranted, including whether cluster characteristics are associated with more aggressive disease.